Update required Ruby version to 3.1.3 in gemspec

Thanks to @patvice for digging that up #240 (comment)

Author: crmne

ruby_llm.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
 
   spec.homepage      = 'https://rubyllm.com'
   spec.license       = 'MIT'
-  spec.required_ruby_version = Gem::Requirement.new('>= 3.1.0')
+  spec.required_ruby_version = Gem::Requirement.new('>= 3.1.3')
 
   spec.metadata['homepage_uri'] = spec.homepage
   spec.metadata['source_code_uri'] = 'https://github.com/crmne/ruby_llm'